EEPROM with split gate source side injection

ABSTRACT

A novel memory structure in which memory arrays are organized in sectors with each sector being formed of a single column or a group of columns having their control gates connected in common. In one embodiment, a high speed shift register is used in place of a row decoder to serially shift in data for the word lines, with all data for each word line of a sector being contained in the shift register on completion of its serial loading. In one embodiment, speed is improved by utilizing a parallel loaded buffer register which receives parallel data from the high speed shift register and holds that data during the write operation, allowing the shift register to receive serial loaded data during the write operation for use in a subsequent write operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 08/639,128, filed Apr.26, 1996, now allowed, which is a continuation of U.S. Ser. No.08/193,707, filed Feb. 9, 1994, now U.S. Pat. No. 5,776,810, which inturn is a divisional of U.S. Ser. No. 07/820,364, filed Jan. 14, 1992,now U.S. Pat. No. 5,313,421, issued May 17, 1994.

TECHNICAL FIELD

This invention pertains to semiconductor memory cells and arrays, moreparticularly to electrically erasable programmable read only memories.

BACKGROUND

Erasable programmable read only memories (EPROMs) and electricallyerasable programmable read only (EEPROMs) are well known in the art.These devices have the ability to store data in non-volatile fashion,while also being capable of being erased and rewritten as desired. EPROMdevices are typically erased by exposing the integrated circuit deviceto ultraviolet radiation, while EEPROMs allow erasure to be performedelectrically.

One form of EEPROM device includes a so-called "split-gate" electrode,in which the control gate includes a first portion overlaying a floatinggate and a second portion directly overlaying the channel. Such a splitgate structure is described in a 5-Volt-Only Fast-Programmable FlashEEPROM Cell with a Double Polysilicon Split-Gate Structure by J. VanHoudt et al, Eleventh IEEE Non-Volatile Semiconductor Workshop, February1991, in which charge is injected into the floating gate from the sourceside of the cell. U.S. Pat. No. 4,652,897 describes an EEPROM devicewhich does not utilize a split-gate, but which also provides injectionto the floating gate from the source side of the device.

As described in the above referenced U.S. Pat. No. 4,652,897, memorycells are typically arranged in an array, as is well known in the art.One form of such an array utilizes buried diffusions, in which sourceand array regions are covered with a fairly thick layer of insulatingmaterial. This is shown for example, in U.S. Pat. Nos. 4,151,020;4,151,021; 4,184,207; and 4,271,421. Such buried diffusion devices oftenutilize a virtual ground approach, in which columns connecting thesources of a first column of memory cells also serves to connect drainsof an adjacent column of memory cells.

While many EEPROM devices utilize two layers of polycrystalline silicon,one for the formation of the floating gate, and the other for theformation of the control gate and possibly electrical interconnects,other EEPROM devices utilize three layers of polycrystalline silicon.For example, U.S. Pat. No. 4,302,766 provides a first polycrystallinesilicon layer for the floating gate, a second polycrystalline siliconlayer for the control gate, and a third polycrystalline silicon layercoupled through an erase window to a portion of the firstpolycrystalline silicon layer for use during erasure of the cell. U.S.Pat. No. 4,331,968 also uses a third layer of polycrystalline silicon toform an erase gate, while U.S. Pat. No. 4,462,090 forms an addressinggate electrode utilizing a third layer of polycrystalline silicon. U.S.Pat. Nos. 4,561,004 and 4,803,529 also use three layers ofpolycrystalline silicon in their own specific configurations.

Japanese Patent Publication 61-181168 appears to utilize three layers ofpolycrystalline silicon to provide additional capacitive coupling to thefloating gate. Japanese Patent Publication 63-265391 appears to pertainto a buried diffusion array, possibly utilizing virtual grounds.

European Patent Application 0373830 describes an EEPROM in which twopolycrystalline silicon layers are used, with the second layer ofpolycrystalline silicon having two pieces, one of which provides theerase function, and one of which provides the steering function.

"A New Flash-Erase EEPROM Cell With a Sidewall Select-Gate on its SourceSide" by K. Naruke et al. IEDM-89-603 and U.S. Pat. No. 4,794,565describe an EEPROM utilizing a side wall select gate located on thesource side of the field effect transistor.

"EPROM Cell With High Gate Injection Efficiency" by M. Kamiya et al.IEDM 82-741, and U.S. Pat. No. 4,622,656 describe an EEPROM device inwhich a reduced programming voltage is provided by having a highly dopedchannel region under the select gate, and the channel region under thefloating gate being either lightly doped or doped to the oppositeconductivity type, thereby providing a significant surface potential gapat the transition location of the channel.

SUMMARY OF THE INVENTION

In accordance with the teachings of this invention, novel memory cellsare described utilizing source-side injection. Source-side injectionallows programming utilizing very small programming currents. Ifdesired, in accordance with the teachings of this invention,to-be-programmed cells along a column are programmed simultaneouslywhich, due to the small programming current required for each cell, doesnot require an unacceptably large programming current for any givenprogramming operation. In one embodiment of this invention, the memoryarrays are organized in sectors with each sector being formed of asingle column or a group of columns having their control gates connectedin common. In one embodiment, a high speed shift register is used inplace of a row decoder in order to serially shift in the data for theword lines, with all of the data for each word line of a sector beingcontained in the shift register on completion of its serial loading. Inone embodiment, additional speed is achieved by utilizing a parallelloaded buffer register which receives data in parallel from the highspeed shift register and holds that data during the write operation,allowing the shift register to receive serial loaded data during thewrite operation for use in a subsequent write operation. In oneembodiment, a verification is performed in parallel on allto-be-programmed cells in a column and the bit line current monitored.If all of the to-be-programmed cells have been properly programmed, thebit line current will be substantially zero. If bit line current isdetected, another write operation is performed on all cells of thesector, and another verify operation is performed. This write/verifyprocedure is repeated until verification is successful, as detected bysubstantially zero bit line current.

Among the objectives of the novel cells constructed in accordance withthis invention are avoidance of programming limitations such as:

1. High Channel Currents (Power) required for Programming.

2. High Drain Voltage Requirements, which increase with increased levelsof erasure.

3. Loss of Read Performance associated with an increase in ProgrammingEfficiency via Heavy Channel doping.

4. Program Wearout Associated with Maintaining a High Drain Bias onCells exposed to this bias, including both those cells targeted forprogramming and those cells not targeted but still exposed to thevoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1b, and 1c, are cell layout, cross sectional diagram, andequivalent circuit schematic of one embodiment of this invention;

FIG. 1d is a plan view of one embodiment of an array consisting of aplurality of cells of FIGS. 1a-1c;

FIG. 1e is a block diagram depicting a memory array organized bysectors, with appropriate control circuitry;

FIG. 1f depicts the operation of one embodiment of a memory arrayorganized by sectors as shown in FIG. 1e;

FIG. 1g is a plan view depicting an alternative array embodimentutilizing cells depicted in FIGS. 1a-1c;

FIG. 2a is a cross-sectional view depicting an alternative embodiment ofthis invention similar that of FIG. 1b;

FIG. 2b is a plan view of one embodiment of an array of memory cellsconstructed utilizing cells depicted in the cross-sectional view of FIG.2a;

FIG. 2c is a diagram depicting the organization and operating conditionof an array such as that of FIG. 2b;

FIG. 3 is a graph depicting the operation of a memory cell of FIG. 1b;

FIG. 4 depicts the electrical field distribution along channels of thedevice of FIG. 5;

FIG. 5 is a cross-sectional view one embodiment of a 2-poly cell of thisinvention;

FIG. 6 is a cross-sectional view of another embodiment of a 2-poly cellof this invention;

FIG. 7a is a plan view depicting a portion of a process sequenceutilized in accordance with one embodiment of this invention;

FIG. 7b is a cross-sectional view of the embodiment shown in the planview of FIG. 7a; and

FIG. 8 is a cross-sectional view depicting a fabrication step suitablefor use in accordance with the teachings of this invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The cell layout, cross-sectional diagram and equivalent circuitschematic of one embodiment are shown in FIGS. 1a, 1b, and 1c,respectively. Similar reference numerals have been used in FIGS. 1a, 1b,and 1c. Referring to the cross-sectional view of FIG. 1b, thisembodiment of the novel EEPROM cell, 101, of this invention includes aburied source region 102 and a buried drain region 103, each beingburied by a relatively thick layer of dielectric 104 and 105,respectively. Channel region 106 is divided into two portions, a firstportion 106-1 which is influenced by the third layer polycrystallinesilicon 109 and which forms a select gate, and a second portion 106-2which is influenced by floating gate 107 formed of a first layer ofpolycrystalline silicon and which, in turn, is influenced by controlgate 108 formed of a second layer polycrystalline silicon. As is wellknown in the art, suitable dielectric layers such as thermally grownoxide are located between channel 106 and polycrystalline silicon layer109 and polycrystalline silicon layer 107. Similarly, suitabledielectric layers such as oxide or composite oxide/nitride are formedbetween the three layers of polycrystalline silicon. Polycrystallinemetal silicide can be used in place of one or more of thepolycrystalline silicon layers 108 and 109. If desired, a highly-dopedP+ region 120 is used within channel 106-2 adjacent buried drain region103. This region 120 is formed, for example, as a double diffused MOS(DMOS) region in order to establish the threshold voltage V_(t) of thememory transistor including channel 106-2. This helps to provide astable threshold voltage, even though the amount of charges trapped inthe gate oxide layer in the vicinity of the gap between 106-1 and 106-2tends to increase with a large number of programming cycles.

An example of operating conditions and levels associated with theembodiment of FIG. 1b are shown in Table 1. High efficiency programmingcomes about by the ability to simultaneously create a high field regionin channel 106-2 under the floating gate, which under the biasconditions of Table 1 occur near the gap between channels 106-1 and106-2 (see above mentioned IEDM article of Kamiya for theory) whilemaintaining a low channel/current. Since this high field causes electroninjection to floating gate 107 near the source side of channel 106-2,this type of operation is termed "source-side" injection. This mechanismprovides high efficiency, low power programming by maintaining a lowchannel current via word line 109 throttling by using a bias operatingnear channel threshold, VT_(p3). A major attribute of this type ofoperation is that it allows for a high drive condition in floating gatechannel 106-2 under the floating gate (in fact it thrives on it),offering high-performance read, without degrading programmingperformance. This is because the very weak drive condition on the selecttransistor of channel 106-1 is established via the throttling mentionedabove to achieve the high fields in the vicinity of the poly 3/poly 1gap. These fields accelerate the electrons to sufficiently energeticlevels (i.e. >3.1 eV) to surmount the Si/SiO₂ interface barrier at thesource side of floating gate 107. Furthermore, there is a significantvertical component to that field (i.e. normal to the Si/SiO₂ surface)driving the electrons up to the surface of channel 106, and therebyassisting the injection into floating gate 107. No read performancepenalty is incurred to establish this high field condition. This is instark contrast to conventional drain side programming, wherein efficientprogram requires strong channel saturation which shuns high floatinggate channel drives, strong overerase, or a weakly turned on seriesselect transistor. These problems with drain side programming dictatehigh channel currents, care in overerase, potentially high drainvoltages, and unfavorable fields (potentially subducting the channelbelow the surface at the drain side and driving electrons downward awayfrom the floating gate).

Consequently, in accordance with the teachings of this invention,programming efficiencies (I_(G) /I_(D)) ranging from 10⁻⁵ to 10⁻³ arepossible, with I_(D) in the range of 1 μA during programming, which istwo to three orders of magnitude smaller than conventional drain sideprogramming. This offers the potential for very fast system levelprogramming by allowing the programming of 100 times as many memorycells in parallel, thereby achieving a 100 fold increase in effectiveprogram speed compared with prior art drain side programming.

                  TABLE 1    ______________________________________    State Table & Operating Conditions (FIG. 1b)              Poly 3   Poly 2    Drain   Source    Node      (Word    (Steering (BN &   & (BN    Operation line)    Gate)     Drain)  Source)    ______________________________________    READ    RELATED    STANDBY   0 v      0 v       1.0 v or 0 v                                         1.0 v or 0 v    READ      5 v      0 v       1.0 v or 0 v                                         0 v or 1.0 v    SELECTED    READ      5 v      0 v       1.0 v   1.0 v    UNSELECTED    ERASE    RELATED    STANDBY   5 v      0 v       0 v     0 v    ERASE Option 1              5 v      -10 to -17 v                                 0 v     0 v    or Option 2              12-22 v  0 v       0 v     0 v    PROGRAM    RELATED    PROGRAM   ˜1.5 v                       14-20 v   5-7 v   0 v    SELECTED    PROGRAM   0 v      14-20 v   5-7 v   0 v    UNSELECTED              ˜1.5 v                       14-20 v   5-7 v   5-7 v              0 v      14-20 v   0 v     0 v    ______________________________________

A major feature of the cell of this invention is the decoupling of theselect function (in this case poly 3 select transistor 110 in FIG. 1b)from the steering function (poly 2 control gate 108). Duringprogramming, this allows the independent control of cell selection/draincurrent throttling via poly 3 word line 109 bias (biased at slightlyhigher than VT_(p3))) and strong positive voltage coupling onto floatinggate 107 (by raising poly 2 control gate 108 to a high voltage, such asabout 12 volts). Also, in accordance with the teachings of thisinvention, the drain voltage can be adjusted independently of steeringand select transistor voltage levels, to optimize programming.

During read, the decoupling feature of this invention provides twoimportant advantages, and one exciting side benefit.

1. The ability to set control gate 108 at the optimum voltage level formemory state sensing, i.e. the best balanced reference point for bothprogrammed and erased states. This independence is in contrast toconventional cells wherein the control gate also serves as the selecttransistor, dictating a voltage level consistent with selection (e.g.Vcc=5v±10%).

2. Improved margin by virtue of being a fixed, (potentially regulated)reference voltage, eliminating the Vcc variation of ±10% inherent to theword line bias levels. (This alone could improve the floating gatememory window by about 0.6v).

3. A side benefit of the ability to independently set the control gatevoltage bias discussed above, offers the possibility of a simple way forre-referencing the memory cell for multi-state (i.e. more thanconventional 2-state) encoded data. For example if the cell is encodedinto three level states, (such as logical 1=strongly erased/highconducting, logical 2=partially programmed/weakly weakly conducting;logical 3=strongly programmed,) then the control gate voltage can be setat two different levels in a two pass read scheme. For example, in thefirst pass read the control gate voltage would be set at about 0v todiscriminate between the logical 1 state and the logical 2/logical 3states. In the second pass read the control/gate voltage is set to about2v, to discriminate between the logical 3 state and the logical1/logical 2 states. By combining the information of this two pass read(e.g. according to Table 2) the original state of the 3 state cell isrecovered. This biasing can be done independently of sense amp referencecell considerations allowing a single sense amp/reference cell circuitto detect the different states via a multi-pass read scheme.

                  TABLE 2    ______________________________________    READ          PASS 1    PASS 2    STATE          Ref. = 0 v!                             Ref. = 2!    ______________________________________    1             Hi        Hi    2             Lo        Hi    3             Lo        Lo    ______________________________________

The two options for erase operation/bias conditions shown in Table 1stem from two different sets of considerations. The first option shownbrings poly 2 control gate 108 to a large negative voltage, but allowspoly 3 word line 109 to remain at a low voltage (e.g. 0v to 5v). This isdesirable since the word lines and their decoders are preferably highperformance, and repeated many times with a tightly pitched requirement,making high voltage word line requirements more difficult and realestate consuming to implement. Poly 2 control or steering gate 108 onthe other hand could be common to a multiplicity of word lines (e.g. asector consisting of 4 or more word lines), putting less demands on realestate and minimal impact to performance. Possible drawbacks of thisapproach are the process and device requirements to support bothnegative as well as positive polarity high voltage circuitry, andreduced steering effectiveness in that the channel cannot assist insteering by virtue of it being held at or near ground (i.e. can't go tolarge negative potential).

The second option of using high word line voltage bias for eraseeliminates both of the above potential drawbacks, but burdens the highperformance, tightly pitched word line/driver with high voltagerequirement.

Note that poly 2 is used only as a steering electrode during all threeoperations. Poly 3, which is the word line connection to the X-decoder,only sees 0V to 5V (other than for erase option 2), and its capacitancecan be made relatively small. It is relatively easy to generate +5V and-17V on poly 2 since both writing and erasing are slow operationsrelative to reading and there is no DC current drain. The -17V doesrequire high voltage PMOS in the erase decode, but the +5V on poly 3aids in reducing the maximum negative voltage required on poly 2 duringerase.

FIG. 1d is a plan view of one embodiment of an array consisting of aplurality of cells constructed as just described with respect to FIGS.1a-1c, and using similar reference numerals. Also shown, are channelstop isolation regions 180.

FIG. 1e shows a block diagram of a memory array similar to that shown inthe plan view of FIG. 1d which is organized by sectors, with appropriatecontrol circuitry. Operation of one embodiment of such a memory arrayorganized by sectors is shown in FIG. 1f. As shown in FIGS. 1e and 1f,in this embodiment sectors are formed by a single column or a group ofcolumns having their control gate connected in common. This allows ahigh speed shift register to be used in place of a row decoder in orderto serially shift in a whole block of column data for the word lines,with the data for each word line being contained in the shift registeron completion of its serial loading. The use of such a high speed shiftregister saves circuit area on an integrated circuit by serving bothencoding and latching functions normally performed by a row decoder.Furthermore, speed is improved by including a parallel loaded bufferregister which receives data in parallel from the high speed shiftregister and holds that data during the write operation. While the writeoperation takes place based upon the data stored in the buffer register,the high speed serial shift register receives the next block of data forsubsequent transfer to the buffer register for the next write operation.In one embodiment of this invention, each sector has an associated latchfor tagging that sector in preparation for an erase of a plurality oftagged sectors.

In one embodiment of this invention, a sector is formed in a group offour cell columns, each column being 1024 bits tall with a commoncontrol gate and an associated sector latch. In this embodiment,verification of programming is performed in parallel on allto-be-programmed cells in a single column. Logical 0 state cells haveword lines at 0 volts while logical 1 state cells have word lines at apositive voltage, such as 5 volts. The control gate and drain voltagesare reduced to a verify level to allow for proper margin testing and thebit line current is monitored. If all of the to-be-programmed cells havebeen properly programmed, the bit line current will be 0 orsubstantially so. If not, it is known that one or more of theto-be-programmed cells in the column have not been properly programmed,and another write operation is performed on the entire column, therebyassuring that any incompletely ones of the to-be-written cells are againwritten. An additional verify step is performed to verify that thecolumn has been properly programmed.

One embodiment of a process suitable for fabricating the structurehaving the cross-sectional view of FIG. 1b is now described. Thisembodiment can be implemented in a very small area with no need for anisoplanar oxide when utilizing a virtual ground, allowing an isolationimplant to be placed in the remaining field which is not covered bydiffusions or polycrystalline silicon and avoids susceptibility tosubstrate pitting associated with the SAMOS etch in the field isolationregion not covered by poly 1. This is achieved, for example, with thefollowing process sequence:

1. Form BN⁺ bit lines in vertical strips. Grow approximately 1500 Åoxide on top of BN⁺, and approximately 200-300 Å gate oxide.

2. As shown in FIGS. 7a and 7b, deposit poly 1 to a suitable conductanceand etch in horizontal strips perpendicular to the BN⁺ diffusion. Fillthe spaces between adjacent strips of poly 1 with deposited oxide, suchas CVD followed by an etch back. This approach protects the fieldisolation regions, and if desired it can be preceded by a boron channelstop implant.

An alternative for steps 1 and 2 of the above process sequence isforming horizontal strips of isolation oxide first, and then depositingP₁ and etched back in RIE to fill and planarize the horizontal groovesbetween adjacent strips of isolation oxide.

3. Form thin dielectric such as ONO of approximately 300-400 Å. coveringpoly 1 strips.

4. Deposit poly 2 and form a suitably thick dielectric overlayer (e.g.,approximately 2000-3000 Å of CVD densified oxide). Etch this oxide andunderlying poly 2 in long vertical strips parallel to bit line (BN⁺)diffusions.

5. Form oxide spacers along edges of poly 2 and use edge of thesespacers to define the floating gate by etching off exposed poly 1 (i.e.poly 1 not covered by poly 2 or by spacer).

6. Form tunnel erase oxide in a conventional manner, as described inU.S. patent application Ser. No. 323,779, filed Mar. 15, 1989, overexposed edges of poly 1 as well as gate oxide over the channel of theselect transistor (channel 106-1 in FIG. 1b).

7. Deposit poly 3 or polysilicide, and form word lines in horizontalstrips.

Another embodiment for achieving a virtual ground cell without the useof the buried diffusion formed early in the process is now described. Inplace of the BN+ of step 1, after step 6 a photoresist masked arsenicsource/drain implant is used, self-aligned to one edge of poly 2 afterpoly 1 stack formation but leaving an unimplanted region along the otheredge to become the poly 3 controlled select transistor channel (see FIG.8). The isolation oxide thickness formed earlier between poly 1 stripsis made sufficiently thick to withstand the self-aligned poly 2/1 stacketch without exposing the substrate to pitting, but thin enough suchthat following this stack etch it is readily removed to expose thesubstrate to the source drain implant. This offers the benefit ofreduced thermal drive of the arsenic junction laterally facilitatingscaling. The remainder of the process steps of this embodiment followsthe prior embodiment.

In summary, the novel cell of this invention offers the followingbenefits.

Very low programming current.

Low programming drain voltage requirement/eliminating associated highvoltage.

Immunity of Programmability to increased levels of erase.

Adjustability of memory state for optimum read of both program anderased states.

Improved margin by elimination of sensitivity to ±10% Vcc variation onthe steering element.

Potential for pure low voltage word line/decoder implementation.

Facilitates multi-state cell sensing.

Reduced susceptibility to source side hot-electron programming inducedtrapping by establishing a separate threshold control region at thedrain.

A second array embodiment is similar to that of FIG. 1d but uses thecell embodiment shown in FIG. 1b, to form a row oriented sectorarchitecture, is shown in FIG. 1g. A sector consists of a group of rows,four in this example, which are erased together. Erase uses option 2 ofTable I, for this row oriented sector architecture, bringing all thepoly 3 word lines of a sector to high voltage. The poly 2 steering gateis common to a group of N sectors where N can range from 1 to the fullsize of the memory array. Similarly the BN+ columns can alternativelycontinuously span the full length of the array or be broken down into acollection of shorter length, local columns. These connect to a global(full array length) column through a select transistor driven by anadditional level of decoding. The local columns can range from 1 to Nsectors. The preferred embodiment is to have local columns span the samenumber of sectors as the poly 2 steering gate. A preferred number ofsectors, N, spanned by local columns and poly 2 steering is around 8.This is because if N is much smaller than 8, the area overhead for localcolumn section devices and poly 2 steering gate routing is high inrelation to the area of arrayed cells, while if N is much larger than 8,the benefits of having localized columns and poly 2 steering diminish.These benefits are: (1) reduced bit line capacitance improving readperformance; (2) reduced repetitive exposure on unselected sectors tothe raised voltage conditions on drains and steering electrodes whenprogramming one sector within the N-sector group, and associatedpotential disturb phenomena; and (3) increased confinement of arrayrelated failures thereby increasing the efficiency of replacing suchfailures. Read, program and unselected conditions are as described inTable I, during read or program. The poly 3 word line in the selectedrow within the selected sector is turned on, 5 volts for read andapproximately 1 volt for programming. Concurrently, the drain to sourcebias conditions are applied to the columns, approximately 5 volts forprogram and approximately 1.0-1.5 volts for read. In one embodiment,alternate bits in a selected row are programmed simultaneously, therebypermitting all bits in a selected row to be programmed utilizing twoprogramming operations. In a similar manner, in this alternativeembodiment, alternate bits in a selected row are read (or verified)simultaneously, thereby permitting all bits in a selected row to be read(or verified) utilizing two read (or verify) operations. After one rowin the sector has finished reading or writing, the next row is selected,and so forth to the end of the sector. The resulting row oriented sectorarchitecture and array operation is much more conventional than thecolumn oriented sector of the first embodiment, and consequentlyoperates in a more traditional manner. Both embodiments share theintrinsic low power capability of this invention, but the row orientedsector embodiment requires, in addition, a full complement of dataregisters to support massively parallel write and verify features.

FIG. 2a shows an alternative array embodiment of this invention whichdoes not utilize buried diffusion regions. Thus, source region 102 anddrain region 103 are formed in a conventional manner and not buried by athick dielectric layer as is the case in the embodiment of FIG. 1b. Aplurality of memory cells are shown in FIG. 2a along a cross section ofa typical array structure, with elements of one such cell numbered usingreference numerals corresponding to similar structure in FIG. 1b. Table3 depicts an example of the operating conditions appropriate for theembodiment of FIG. 2a. This a more traditional cell approach compared tothe buried diffusion cell, with source/drain diffusions formed after allthe polycrystalline silicon structures are formed. It requires one draincontact to metal bit line for every 2 cells, making it approximately 30%to 50% larger than the buried diffusion cell with similar layout rules.In all other respects, this alternative embodiment offers the samebenefits as listed above for the buried diffusion embodiment of FIG. 1b.

FIG. 2b is a plan view of one embodiment of an array of memory cellsconstructed as described above with reference to FIG. 2a.

FIG. 2c is an equivalent circuit diagram depicting the organization ofsuch a memory array in sectors, with appropriate operating conditionsand voltages shown. The preferred embodiment for a sector organizedarray uses two word lines which straddle a source line as part of asector, along with their associated poly 2 steering gates and sourceline. A full sector consists of some multiple of such pairing (e.g. 2such pairs or 4 word lines, each word line containing 128 bytes andoverhead cells, and straddling two source lines, constitute one sector).

As shown in the embodiment of FIG. 2c, the steering lines are connectedtogether within a sector as are the source lines (i.e. a sector whichconsists of row lines grouped together respectively and driven by commondrivers). The embodiment described here confines the write operation tothe sector being written to, while the bit line bias conditions (2.5vduring read and approximately 5v possible during write) arenon-disturbing to the cells because the bias is applied to the selecttransistor side of the cell and not to the floating gate side. In a twostate cell, to write the cell to a logical one, the bit line is held atzero volts, causing the cell to program via source-side injection.Conversely, to inhibit writing, the bit line is held high (typicallyabout 5 volts), thereby cutting off the channel, leaving the cell in theerased state.

Sector erase takes place by tagging the selected sector and raising theassociated row lines to a sufficiently high voltage to erase thefloating gates to their required erased levels.

Because of the low programming currents associated with source sideinjection (approximately 1-5 microamps/cell), massive parallelprogramming is made practical, e.g. a full row line of approximately1000 cells is programmed in a single operation with total current lessthan approximately 1-5 mA, thus providing more than 100 times moreefficiency than prior art drain side programming arrays.

                  TABLE 3    ______________________________________    State Table & operating Conditions (FIG. 2a)                 Poly 3   Poly 2                 (Word    (Steering    Node Operation                 line)    Gate)      Drain Source    ______________________________________    READ RELATED    STANDBY      0 v      0 v        Don't 0 v                                     care    READ SELECTED                 5 v      0 v        2.5 v 0 v    READ UNSELECTED                 5 v      0 v        Don't 0 v                                     care    ERASE RELATED    STANDBY      0 v      0 v        0 v   0 v    ERASE    Option 1     12 v-22 v                          0 v        0 v   0 v    Option 2     5 v      -10 v to -12 v                                     0 v   0 v    PROGRAM RELATED    PROGRAM SELECTED                 ˜1.0 v                          14-20      0 v   5 v-8 v    PROGRAM      0 v      14-20      0 v   5 v-8 v    UNSELECTED   ˜1.0 v                          14-20      5 v   5 v-8 v                 0 v      14-20      5 v   5 v-8 v    ______________________________________

FIG. 3 is a graph depicting the gate current into poly 1 gate 107 ofFIG. 1b (which is not floating in the FIG. 3 test device to allow thismeasurement to be made) as a function of poly 1 gate voltage(V_(poly) 1) while keeping the select transistor 110 V_(p2) at justabove its threshold. In this way most of the potential drop in channel106 of FIG. 1 occurs in channel portion 106-1 underneath gate 109 ofselect transistor 110, and electrons accelerated in this channel arethen injected onto floating gate 107. From FIG. 3 it is seen the hotelectron programming injection efficiency of this device is phenomenallyhigh.

Various embodiments of a process suitable for fabricating a structure inaccordance with the embodiment of FIGS. 1a-1d are now described.Reference can also be made to copending U.S. application Ser. No.323,779 filed Mar. 15, 1989, and assigned to SunDisk, the assignee ofthis invention. Reference may also be made to fabrication process stepsdescribed earlier in this application. A starting substrate is used, forexample a P type substrate (or a P type well region within an N typesubstrate). A layer of oxide is formed, followed by a layer of siliconnitride. The layer of silicon nitride is then patterned in order toexpose those areas in which N+ source and drain regions are to beformed. The N+ source and drain regions are then formed, for example, byion implantation of arsenic to a concentration of approximately 1×10²⁰cm-3. The wafer is then oxidized in order to form oxide layers 104 and105 in order to cause source and drain regions 102 and 103 to become"buried". Note that for the embodiment of FIG. 2a, this oxidation stepis not utilized, as the source and drain regions are not "buried". Theremaining portion of the nitride mask is then removed, and the oxideoverlying channel regions 106-1 and 106-2 is removed. A new layer ofgate oxide overlying channel regions 106-1 and 106-2 is formed, forexample to a thickness within the range of 150 Å to 300 Å and implantedto the desired threshold (e.g. approximately -1v to +1v).Polycrystalline silicon is then formed on the wafer and patterned inorder to form floating gate regions 107. If desired, the polycrystallinesilicon layer is patterned in horizontal strips (per the orientation ofFIG. 1a), with its horizontal extent patterned at the same time as thepatterning of the second layer of polycrystalline silicon, as will benow described. Following the formation polycrystalline silicon layer 107at this time, a layer of oxide or oxide/nitride dielectric is formedover the remaining portions of polycrystalline silicon layer 107. Asecond layer of polycrystalline silicon 108 is then formed and doped toa desired conductivity, for example 30 ohms/square. The second layer ofpolycrystalline silicon is then patterned into vertical strips (again,per the orientation of FIG. 1a). If the horizontal extent ofpolycrystalline silicon layer 107 was not earlier defined, this patternstep is also used to remove the layer of dielectric between the firstand second layers of polycrystalline silicon in those areas where thefirst layer of polycrystalline silicon is to be patterned simultaneouslywith the patterning of the second layer of polycrystalline silicon.Following the first layer patterning, an additional layer of dielectricis formed on the wafer to form the gate dielectric above channel region106-1, and above any other areas in the silicon substrate to which thethird layer of polycrystalline silicon is to make a gate. These regionscan then be implanted to the desired threshold voltage (e.g.approximately 0.5v to 1.5v). The third layer of polycrystalline siliconis for a transistor (ranging from ˜200 Å to 500 Å in thickness) thenformed and doped to appropriate conductivity, for example 20ohms/square. Polycrystalline silicon layer 109 is then patterned inorder to form word line 109.

In one embodiment of this invention, polycrystalline silicon layer 107is patterned to form horizontal stripes and channel stop dopants (e.g.boron) are implanted into the exposed areas therebetween in order toform high threshold channel stop regions between adjacent rows of amemory array.

The thickness of the gate dielectric between channel 106-2 andpolycrystalline silicon floating gate 107 can range from approximately150 angstroms or less to approximately 300 angstroms or more, dependingon performance tradeoffs. For increased drive for reading, a thinnergate dielectric is desired while for increased coupling betweenpolycrystalline and silicon control gate 108 and floating gate 107(helpful during programming) a thicker gate dielectric is desired.

Second Embodiment

FIG. 5 is a two-poly embodiment in which programming occurs by takingdrain 303 high, for example about 10V while raising control gate 308just sufficiently so as to turn on select transistor 310. Since thisV_(CG) voltage can vary from one device to another it is possible toachieve the optimum injection conditions by keeping V_(CG) at about 3Vwhile raising source (virtual ground) 302 in a sawtooth fashion fromabout 0 to 3 volts and back to 0 again, with a period on the orderapproximately 1 microsecond.

This ensures that at some point along the sawtooth the optimum injectionconditions are met. Reference can also be made to European PatentApplication Serial No. 89312799.3 filed Aug. 12, 1989. To furtherenhance programming efficiency, in one embodiment a programmingefficiency implant 330 (shown in dotted line) is introduced at thesource side. To read the device, its source is 0V, drain isapproximately 1.0v and V_(CG) approximately 4.5-5v. To erase we employpoly 1-poly 2 tunneling between floating gate 307 in word line 308 atthe tunneling zone, consisting of one or more of the floating gateedges, sidewall, corners of the top edge, portions of the top andportions of the bottom, of floating gate 307, associated with a tunneloxide (400 Å-700 Å). Erase takes place with V_(CG) approximately 12-22V,V_(D) =0V, V_(S) =0V. A capacitive decoupling dielectric (approximately1500 to 2000 Å thick) 340 is formed on top of poly 1 to reduce thecapacitance between poly 1 and poly 2.

In one embodiment of this invention, a high electrical field region iscreated in the channel far away from the reverse field region located inconventional devices near the drain. This is achieved, for example, byutilizing region 330 of increased doping concentration at the boundarybetween channels 306-1 and 306-2 under floating gate 307. In oneembodiment, the width of region 330 is on the order of 0.1 microns. Alarger dimension for region 330 can be counterproductive, reducing theselect transitor drive with no gain in efficiency.

FIG. 4 depicts the electrical field distribution along channels 306-1and 306-2 in structures with and without poly-doped region 330. In astructure without region 330 and improperly biased select transistor theelectron injection can take place in the high field region near drain303. Because of the transversal field reversal region near drain 303,the resultant injection efficiency is reduced. In a structure withregion 330 the injection takes place in the high field region located atregion 330, far away from the field reversal region. Because of this,increased injection efficiency is achieved.

From the processing side there are three problems which must beaddressed properly:

1. The formation of sufficiently thin/high quality gate dielectric overBN+, which tends to oxidize more quickly than undoped silicon.

2. The misalignment between poly 1 and the buried N+ drain diffusionstrongly affects the coupling ratios for programming and erase. This canbe overcome at the expense of an increase in cell area by not using avirtual ground array, but instead a shared source array.

3. This array permits floating gate 307 to completely overlap the buriedN+ diffusion in a dedicated source arrangement, eliminating thisalignment sensitivity. Unfortunately, this array requires an extraisolation spacing adjacent to the BN+ to prevent the poly 1 extensionbeyond BN+ in the direction away from channel 306-2 to form a transistorin the neighboring cell.

To achieve small cell size in the buried diffusion direction a channelstop isolation is used between adjacent cells, plus a self-alignedstacked etch to simultaneously delineate poly 2 and poly 1. This isdifficult to do without pitting the substrate as well as the exposed BN+when etching the exposed poly 1 between adjacent cells. This isespecially difficult to avoid when etching the decoupling oxide(1500-2000 Å thick on top of poly 1 in order to expose poly 1, since thesubstrate unprotected by poly 1 also becomes exposed, so that when poly1 is etched, the substrate in those regions becomes pitted.

This will therefore require formation of a thick dielectric region aspart of the field isolation process protecting the substrate in thespace between the poly 2 word lines. This can be accomplished by using aprocess as described in U.S. patent application Ser. No. 323,779, filedMar. 15, 1989, and assigned to SunDisk, the assignee of thisapplication. This is actually forming trench isolation, but with BN+abutting this trench, we may experience severe junction leakage as wellas loss of a portion of the BN+ conductor. This cell of this secondembodiment is attractive because it is double poly, low programmingcurrent, very fast programming, programming away from drain junction,small and scalable cell. Cell size is quite attractive as indicatedbelow for three representative geometries:

1.0μgeometries: cell=4.0×2.0=8.0μ²

0.8μgeometries: cell=3.2×1.6=5.2μ²

0.6μgeometries: cell=2.3×1.2=2.8μ²

Third Embodiment

FIG. 6 is a cross-sectional view of alternative embodiment of a two polycell, using source side injection for programming, aided by strongcoupling to buried N+ drain 403, which acts also as a second controlgate. Erase is by Fowler-Nordheim tunneling to channel 406 through asmall thinned oxide region, formed for example to a thickness of about100 Å, by utilizing a thin polyspacer. These process steps would be asfollows: Once the drain oxide is formed (i.e. the oxide above drain403), a first layer of poly, (approximately 2000 Å to 4000 Å thick) isdeposited and a thin nitride dielectric is deposited on top. Theselayers are then etched using a poly 1 mask to delineate the lateralextent (as shown in FIG. 6) of the poly 1. The exposed oxide layer overthe channel portion of the substrate is then stripped and regrown to the100 Å thickness required for tunneling, while a photoresist maskedpattern protects oxide over the exposed, BN+ side of the poly 1 frombeing stripped. The nitride overlayer on poly 1 prevents oxide fromforming on that poly. The thin nitride is then etched off using a highlyselective etch which does not attack or degrade the 100 Å tunnel oxide(e.g. hot phosphoric or plasma etch). This is followed by a second polydeposition which electrically contacts the first poly on its topsurface. This structure is then etched using an anisotropic poly-siliconetch, with etch being terminated with the re-exposure of the oxidelayers over substrate beneath the second deposited poly layer. Thiscompletes the formation of the poly 1 floating gate stripe shown in FIG.6. The remaining process is similar to that of the second embodiment.

In this embodiment, programming is from hot channel electrons injectedfrom grounded source diffusion 402 with drain 403 held at about +8v andfixed control gate of around 1.5v. Alternatively, programming isperformed by hot channel electrons from source diffusion 402 utilizing asawtooth control gate voltage ranging from 0 volts to a peak voltageapproximately 3 volts, as described previously for the secondembodiment. Read is achieved with V_(DS) =1.5V, Vs=0, V_(CG) =+5V. Eraseis achieved with V_(CG) =-22V, Vs=Vd=OV. In this embodiment, the poly 2word line 408 will carry the +5 volts during read and the -22 voltsduring erase, thereby requiring an X-decoder capable of serving thispurpose. Coupling considerations require that C_(P2P1) >C_(P1D), whichis unfavorable for programming. Therefore the cell must be optimized forbalancing erase against programming by adjusting oxide thicknesses andfloating gate threshold to the optimum print. There is less of a problemwith pitting the field regions between cells in the poly 1 direction(because poly 1--poly 2 oxide or ONO is thin). This may obviate the needfor the additional thick oxide field region described for the secondembodiment. However, there is the additional process complexity offorming the thin oxide region and extra space needed to place this thinoxide region sufficiently far from the source diffusion.

Alternative Operating Methods

A number of alternative methods are possible to program the source sideinjection cells described in the previous embodiments. Strong capacitivecoupling (for example, using thin ONO) is required in the second andthird embodiments between poly 2 and poly 1 for programming. Duringoperation, one embodiment applies V_(D) at 5 to 7v, V_(S) =O, thecontrol gate voltage V_(CG) is raised to just turn on the control gatechannel, and V_(p2) is on the order of about 12 volts or more.Alternatively, the source body effect is used to advantage. In thisalternative embodiment, rather than bringing control gate to a specifiedvalue to just turn on the channel, the control gate is brought to avalue greater than the voltage required to just turn on the channel(e.g., approximately one volt above) and a pull-down circuit is used(e.g., a high impedance resistor or a current sink) for providingapproximately 1 μcurrent flow via source debiasing. Alternatively, thecontrol gate voltage V_(CG) can be operated in a sawtooth fashion frombetween 0 volts to about +3 volts, as mentioned previously with respectto European patent application serial number

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

The invention now being fully described, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the appendedclaims.

What is claimed is:
 1. A memory array comprising:a plurality of adjacentbit lines running in a first direction to form a corresponding pluralityof columns; a plurality of word lines running in a second directiongenerally perpendicular to said first direction to form a correspondingplurality of rows; a plurality of memory cells, each memory cell beingassociated with the intersection of one of said bit lines and one ofsaid word lines; a shift register comprising:an data input port forreceiving data to be stored by a selected one of said memory cells; aplurality of storage elements, each associated with one of said wordline, for storing said data to be stored by said memory cells; andcontrol circuitry for serially receiving a plurality of data values fromsaid input port, each said data value associated with one of said wordlines, and storing said plurality of data values for writing intocorresponding ones of said plurality of memory cells along a selectedbit line; and programming circuitry for establishing voltages on saidbit lines to cause one or more of said data values stored in saidplurality of storage elements to be written into selected ones of saidmemory cells along said selected bit line.
 2. A memory array as in claim1 wherein said data values are applied to said word lines for writing.3. A structure as in claim 1 which further comprises a plurality ofsegments, each segment including a subarray.
 4. A structure as in claim1 which further comprises a plurality of steering lines running in saidfirst direction.
 5. A structure as in claim 1 which further comprises aword line buffer register comprising:a parallel input port coupled to aparallel output port of said shift register; and a parallel output portcoupled to said plurality of word lines.
 6. A structure as in claim 5wherein said shift register reads data values serially in while saidword line buffer stores previously received data values for writing tosaid memory cells.